This invention relates to a superconducting apparatus, and particularly to a superconducting apparatus including a current lead penetrating through a thermally insulating vessel.
FIGS. 1 and 2 show the structure of a conventional, typical super conducting magnet wherein a superconducting coil 1 is supplied with current through a current lead 2 and a connecting lead 3. A thermally insulating vessel 4 for enclosing a cryogenic liquid to cool a superconducting coil 1 comprises a vessel body 4a and a lid 4b. The lid 4b mechanically fixed on the vessel 4 shields the atmosphere inside the vessel from the outside atmosphere. The current lead 2 penetrates into and is supported by the lid 4b which is also provided with piping and the like (not shown). Liquid helium 6 is stored within the vessel body 4a. Immersing the superconducting coil 1 into liquid helium cools the coil 1 to cryogenic temperatures (about -269 degrees Celsius) and a superconducting state. Helium gas 7 fills the upper space of the vessel 4. A plurality of baffle boards 8 are provided to reduce the heat penetration from the lid 4b.
The withstand voltage insulation between the top portion of the current lead 2 and the lid 4b is maintained by an insulating ring 9 disposed in a normal temperature region and a gasket 10 mounted between the current lead 2 and the insulating ring 9. The insulating ring 9 and the lid 4b also form a seal between the atmosphere inside the vessel and the outside air. Insulating papers 11 and 12 are respectively is wound around the current lead 2 and the connecting lead 3 to maintain the withstand voltage insulation thereof. The unnumbered arrows indicate the direction of flow of helium gas 7. The current flowing into the current lead 2 causes a resistance loss. Generally, the current lead 2 has a bore (not shown) extending therethrough for cooling the current lead 2 by the flow of helium gas through the bore.
However, in the prior art construction of the current lead 2, the insulating ring 9 and the insulating paper 11 are distinct members separated from one another, so a portion of the current lead 2 is exposed to warmed helium having a low withstand voltage, and electrical discharges sometimes occur between the current lead 2 and the lid 4b. Furthermore, in the conventional superconducting magnet of the above construction, the insulating ring 9 is disposed at a sealing portion between the inside atmosphere of the vessel and the outside atmosphere, and the withstand voltage of helium gas 7 inside the vessel is only a fraction of the withstand voltage of air at normal temperatures (about 20 degrees Celsius). Therefore, if the helium temperature inside the vessel near the insulating ring 9 rises up to the temperature (about 20 degrees Celsius) of the outside atmosphere, electrical discharges may occur between the current lead 2 and the lid 4b, and between the current lead 2 and the baffle boards 8.
Furthermore, since the connecting lead is covered by the insulating paper 12, there is the disadvantage that the connecting lead 3 is prevented from heat radiation and the temperature thereof rises.